This invention relates to apparatus and method for converting analog stroke display signals to raster display information for developing a raster-scan image display.
Computer graphics involves the development of images for display on some type of display device. The display devices developed and used initially in computer graphics were known as a vector, stroke or line drawing displays. (The terms xe2x80x9cvectorxe2x80x9d and xe2x80x9cstrokexe2x80x9d are synonymous with xe2x80x9clinexe2x80x9d.) To develop characters for display, the character outlines were composed of sequences of strokes, produced by moving an electron beam of a cathode ray tube (CRT) to draw the character on the phosphor coating of the CRT screen. The electron beam is deflected, to draw the character, by beam-deflection circuits in response to analog voltages developed by a vector or stroke generator.
The stroke generator of a stroke display device typically develops an analog X major deflection signal and an analog Y major deflection signal for moving the electron beam from one general location to another. Some stroke generators also developed an additional analog x character deflection signal and an additional analog y character deflection signal for guiding the electron beam within the general location to which the beam has been moved, to draw or trace a line, character, symbol, etc.
The final signal typically developed by a stroke generator is a three voltage level video signal, having a 0 volt level to indicate that the electron beam is off (a condition where the beam is to be moved from one location to another without producing a trace), a 1 volt level defining a certain brightness of the trace when moving the beam from one general location to another to produce a trace, but not to draw a character, and a 2 volt level for increasing the brightness of the trace when drawing a character after the beam has been moved to a desired character location. The brightness is increased when drawing a character since the movement of the beam is much faster than when simply moving the beam from one location to another; the voltage level must be boosted just to maintain the brightness of the drawn character to about the same as the brightness of the trace.
The next major development in computer graphics was that of raster graphics or raster display devices, which were based on television technology. Raster displays store so-called display primitives (such as lines, characters, and solidly shaded or patterned areas) in a refresh buffer, in the form of component pixels. The image on a raster display device is formed from a set of horizontal raster lines, each line containing a row of individual pixels. The xe2x80x9crasterxe2x80x9d is stored in a frame buffer as a matrix of pixels representing the entire screen (frame) area. The image is scanned out sequentially by a video controller, one raster line at a time, from top to bottom and then back to the top, in the same manner as a television picture or image is created. Each pixel includes information to determine the electron beams intensity when being moved through that pixel location on the screen. All this is well known technology and a full description and discussion is given in Foley, James D. et al., Computer Graphics Principles and Practice, Addison-Wesley Publishing Company, 1996.
Raster displays have a number of advantages over stroke displays including the need for less expensive image generating logic. Also, raster displays can fill areas with a uniform color or a repeated pattern in two or more colors, and can store images in a way that allows manipulation at a fine level, including the copying or moving of selected portions of an image.
One disadvantage of a raster display, compared to a stroke display, results from the discrete nature of the pixel representation. Primitives are specified in terms of their end points (vertices) and must be scan-converted into their component pixels in the frame buffer. Because each primitive must be scan-converted, real-time processing is computationally more complicated for raster displays than for stroke displays. Nevertheless, most computer graphics systems today utilize raster displays.
Because large numbers of stroke display systems had been installed in the past and many are still in use today (for example, stroke display systems are common in commercial and military aircraft), it has been found desirable, when upgrading an existing stroke display system, to save as much of the existing circuitry as possible, especially the stroke generator circuitry. As a result, stroke to raster converters have been developed to enable use of the stroke generator, but to convert the stroke generation signals to raster producing signals for use on a raster display device. In effect, a raster display replaces the stroke display to develop the desired images.
The prior art approach to converting from stroke to raster displays involved sampling the analog stroke signals to obtain data indicating the brightness of the stroke trace, and then adding the samples for corresponding similar pixel locations to obtain a composite pixel representation of the stroke trace. A problem with this approach is that for crossing or intersecting traces, the trace would be sampled twice where the trace is crossed or intersected, and the pixel representation of such crossing or intersection would be an excessively bright spot. Of course, if a number of characters are displayed on the screen, each with a number of line intersections, then the image would be quite disconcerting when viewed because of the number of excessively bright spots.
It is an object of the invention to provide apparatus for converting a stroke video signal in real-time to appropriate pixel and sub-pixel data for generating raster video display.
It is another object of the invention to provide a stroke to raster converter which provides high resolution in the resulting raster display.
It is also an object of the invention to provide such a converter which is easy and relatively inexpensive to implement.
It is a further object of the invention to provide such a converter, in accordance with one aspect thereof, for performing anti-aliasing during the conversion process.
It still another object of the invention, in accordance with another aspect thereof, to provide such a converter in which xe2x80x9cweightingxe2x80x9d of sub-pixels is employed to present improved definition of lines and objects presented for display.
The above and other objects are realized in an illustrative embodiment of apparatus for converting stroke display signals (representing electron beam generated stroke traces) into raster display information for developing a raster video image display. The apparatus includes a sampling circuit for sampling the analog stroke display signals to produce pixel data representing pixel locations covered or touched by the stroke traces, a frame buffer for temporarily storing the pixel data samples for subsequent supply to a raster display device.
In accordance with one aspect of the invention, the sampling circuit is adapted to sample at a rate sufficient to allow development of digital representations of pixels subdivided into arrays of sub-pixels with indications of which sub-pixel locations were covered or drawn by the stroke trace.
In accordance with another aspect of the invention, a filter, is provided between the frame buffer and the display device for calculating brightness of pixels based upon the amount of A coverage of the pixel by the stroke trace.